Aspects of this topic are discussed in the following places at Britannica.
Great circles are the “straight lines” of spherical geometry. This is a consequence of the properties of a sphere, in which the shortest distances on the surface are great circle routes. Such curves are said to be “intrinsically” straight. (Note, however, that intrinsically straight and shortest are not necessarily identical, as shown in the figure.) Three intersecting...
Spherical trigonometry involves the study of spherical triangles, which are formed by the intersection of three great circle arcs on the surface of a sphere (see the figure). Spherical triangles were subject to intense study from antiquity because of their usefulness in navigation, cartography, and astronomy. (See the section Passage to Europe.)
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